Plant-derived oil can be produced from seeds which contain, oil, and from oleaginous fruits such as palm fruits. The production of fruit oil is most generally practiced by using a wet pressing method. As a side fraction, wet pressing yields a colloidal suspension, consisting of water-soluble components of fruit and a material suspended in water, particularly fibers and oil. In palm oil production, this colloidal suspension, palm oil mill effluent (POME), contains typically 95 to 96% water, about 1% oil, and has a total dry content of 4 to 5%, of which the suspended matter makes up 2 to 4%. Main sources of solids include sterilizer condensate, separator sludge or oil clarifier, and hydrocyclone effluent.
Even on the basis of its composition, it is possible to conclude that POME produces quite a considerable biochemical and chemical effluent load. It has been presented in the literature that the chemical oxygen demand (COD) of POME varies typically within the range of 30 to 100 gCOD/L and the biological oxygen demand, in turn, is typically about 25 to 60 gBOD/L. The palm oil wet pressing process consumes plenty of water and produces a large quantity of POME. A few properties of POME are presented in the following table 1.
TABLE 1Properties of POME.PropertySample1Sample 2pH3.6BOD53-57 g/l  38-39 g/l  CODsoluble31 g/l19 g/lCODtotal84 g/l43 g/lDry matter40 g/l(volatile suspended solids)Ntotal1.1 g/l 0.4 g/l Ptotal0.11 g/l  0.98 g/l  
Depending on the pressing process, the output of POME ranges from one ton to as much as four tons per one ton of palm oil mass produced, or from half a ton to one ton per ton of fruit bunches processed. Prior to its subsequent effluent treatment, POME is typically acidic and has its pH varying within the range of 3.0 to 6.5.
The production of palm oil has increased rapidly all over the world. Most important uses of palm oil include food applications (about 80% of the produced oil), the production of oleochemicals and soaps (about 15%), as well as the production of biofuels (less than 5%). As the production of palm oil is highly extensive worldwide, about 40 million tons a year, the amount of resulting POME is respectively as large as more than 100 million tons a year.
Regarding the treatment of POME, there have been proposed numerous, even large-scale adaptable methods, such as anaerobic treatment, aerobic microbial processes, and bioreactors based on membrane technology. Various methods for the treatment of POME have been described in the article Poh P E, Chong M F. 2009. (Development of anaerobic digestion methods for palm oil mill effluent (POME) treatment. Bioresource Technology 100: 1-9). Goh et al. 2010 have proposed palm-based biofuel refinery (PBR) and production of renewable biofuel, such as biodiesel and bioethanol from CPO and lignocellulosic residues (Goh C S, Lee K T. Palm-based biofuel refinery (PBR) to substitute petroleum definery: An energy and emergy assessment. Renewable and sustainable energy reviews. Vol. 14, No. 9, December 2010, pages 2986-2995). Wicke at al. 2008 have analysed the greenhouse gas emissions (GHG) of crude palm oil (CPO) and palm fatty acid distillate (PFAD) production and their co-firing with natural gas for electricity production. They have described also the conversion of crude palm oil to biodiesel and the associated GHG emissions (Wicke B, Dornburg V, Junginger M and Faaij A. Different palm oil production systems for energy purposes and their greenhouse gas implications. Biomass and bioenergy 32 (2008)1322-1337). Typically, the treatment of POME is conducted at palm oil mills by using anaerobic as well as aerobic effluent tanks and combinations thereof for reducing the organic load of POME to an acceptable level. The organic matter of POME produces methane upon degradation in anaerobic conditions in anaerobic ponds. As for now, the resulting methane is not always collected from anaerobic ponds and it escapes into the atmosphere. This is a source of major greenhouse gas emissions as methane is a greenhouse gas 23 times more powerful than carbon dioxide. In light of these figures, it is obvious that the treatment of POME on principles that are environmentally friendly and comply with sustainable development is a remarkable challenge.
Accordingly, there is a particular need for a treatment method, which enables both recovering oil from the effluent of a fruit pressing plant and concurrently reducing the environmental burden of this effluent.
Sand filtration is included in prior known POME purifying treatments, used especially as a pretreatment method in combination with other post-treatment techniques. Prior known is a technique (Wong, P. W., Nik, M. S., Meenakshisundaram, N., Balaraman, V. Songklanakarin J. Sci. Technol., 2002, 24:891-898), wherein sand filtration is applied as a pretreatment of POME prior to the actual ultrafiltration-based purifying process. However, the method description does not include processes which would indicate a continued treatment of the fraction obtained in sand filtration for separating oil from filtration residue, and, therefore, the method specifically does not comprise a process in which the use of sand filtration is repeated for the recovery of oil.
Sand filtration is used not only for the purification of POME but also other oil-water suspensions. Publication EP0062527 A2 (1982-10-13) discloses a method of separating oil from water emulsion by filtering it through silylated sand. The method also describes a step of separating oil from sand by means of reflux washing.
Sand filtration is an economically feasible option for the separation of oil from water, especially when there is a low concentration of oil or oleiferous components. This type of method is known from publication JP57136989 A (1982-08-24), which describes the purifying of waste water by using a sand bed with adjuvants added therein.
The use of organic solvents is known for the removal of palm oil from POME. In publication GB2023120 (1979-12-28), the object of separation is POME as such, meaning that there are immense volumes of aqueous solution to be extracted.